raft: accept snapshots transfer from multiple nodes simultaneously
raft: accept snapshots transfer from multiple nodes simultaneously

A leader may change while one of its followers is in snapshot transfer
mode and that node may get additional request for snapshot transfer from
a new leader while previous transfer is still not aborted. Currently
such situation will trigger an assert. This patch allows to have active
snapshot transfers from multiple nodes, but only one of them will succeed
in the end, all other will be replied to with 'fail'.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla